Many fuel and emission control systems for automobiles use flexible diaphragms to control fuel-air mixtures. Depending on the actual design these diaphragms, which are usually fabricated from an elastomeric material, are supported by a metal insert that serves either to position the diaphragm or which interacts with a sensor in response to movement of the diaphragm. Typically these metal parts are manufactured from galvanized steel but other metals such as stainless steel, aluminum and titanium have been used. Occasionally mineral reinforced polymers such as mineral filled nylon have been used as supporting inserts for elastomeric diaphragms in fuel control devices. The elastomeric material forming the diaphragm must bond to the supporting substrate, such as a metal insert, with a sufficiently strong bond that the mode of failure of the elastomeric diaphragm is cohesive rather than adhesive. This means that the primary failure mode of the device comprised of the metal insert and the elastomeric diaphragm is within the diaphragm (cohesive) rather than a failure of the diaphragm metal bond (adhesive).
There are generally two methods of constructing such composite devices. The first method involves incorporating an adhesion promoter into the elastomeric composition so that when the diaphragm is formed in situ in the presence of a supporting metal insert it will bond to the metal. This causes fusion between the elastomer and the mold because most of the molds are metal and the elastomer does not distinguish between the metal of the insert and the metal of the mold. Consequently when a formed in place molding process is used, the mold must be lined with Teflon.RTM. or Mylar.RTM. in order to prevent the elastomer from adhering to the metal of the mold.
An alternative approach is to use a primer on the metal insert. Thus when manufacturing a formed in place diaphragm, the metal insert is coated with a primer and the elastomer bonds to the primed metal and is released from the unprimed metal of the mold. Commercial primers to accomplish this are available but a significant drawback of these commercial formulations is that the solvents employed are usually mixtures of volatile aliphatic and aromatic hydrocarbons. The solvent in the primer composition must be volatile in order to evaporate quickly and form a primer layer on the metal substrate. Because the volatile solvents used in commercial formulations are organic and volatile there is a concern about emissions of volatile organic compounds (VOC). This has led to a desire for primer formulations that do not contain volatile organic compounds.